Electrostatic chucks, also known as susceptors, are employed to support various substrates, such as wafers, during the manufacture of semiconductor devices. Electrostatic chucks often are fabricated from ceramic materials. Among these, aluminum nitride (AIN) ceramics have a number of attractive properties such as, for example, good shock resistance, good thermal conductivity, good thermal expansion match to silicon, and good erosion and corrosion resistance to plasma.
Both coulombic and Johnson-Rahbek types of chucks can be employed in wafer manufacturing. To operate in the Johnson-Rahbek mode, the volume (or dielectric) resistivity of the chuck typically is in a range from about 108 to about 1013 ohmxc2x7cm at operating temperature and preferably in a range from about 109 to about 1011. For instance, in a dry etch process, where the wafers are processed near room temperature, the chuck generally should exhibit a resistivity of 109-1011 ohmxc2x7cm at room temperature. For vapor deposition processes (PVD and CVD), the chuck exhibits the 109-1011 ohmxc2x7cm volume resistivity range at higher temperatures, for example, from about 250xc2x0 C. to about 700xc2x0 C.
The volume resistivities of aluminum nitride ceramics, however, generally tend to be higher than those considered suitable for fabricating Johnson-Rahbek electrostatic chucks. For example, at room temperature, the volume resistivity of an aluminum nitride ceramic body, in the absence of dopants, generally is above about 1013 ohmxc2x7cm.
Methods employed to lower the volume resistivity of dense aluminum nitride bodies include adding small amounts of metals or carbon to the aluminum nitride material. In wafer manufacturing, however, impurities present in the chuck body can contaminate the wafer and are generally undesirable.
Impurities can also be introduced into AIN ceramics in the form of sintering aids used to promote densification during conventional (pressureless) sintering. Examples of sintering aids include yttrium oxide (Y2O3), calcium fluoride (CaF2), calcium oxide (CaO) and calcium carbonate (CaCO3). Typical amounts employed are generally in the range of 0.1 to 3 weight percent. One disadvantage associated with the presence of sintering aids is the formation of metal-aluminate phases such as, for example, yttrium aluminate. During wafer manufacturing, such phases can be attacked by plasma and can generate undesirable contamination of the wafer.
One process for forming an electrostatic chuck employs a raw AIN material having a metal content, other than aluminum, of less than about 100 ppm. The raw material is sintered in an inert atmosphere, such as nitrogen. However, the extent to which volume resistivity can be lowered by such methods generally has been limited.
One method of modifying the dielectric properties of an aluminum nitride body employs conventional (pressureless) sintering and cooling under an argon atmosphere. Since as much as a few percent by weight of Y2O3 is added as a sintering aid, to achieve densification, the resulting material is not high in purity and can exhibit undesirable yttrium aluminate phases. Furthermore, the room temperature volume resistivity of electrostatic chucks fabricated by this method generally are too high for Johnson-Rahbek applications.
Therefore, a need exists for a high purity aluminum nitride ceramic having a volume resistivity suitable for the manufacture of Johnson-Rahbek type electrostatic chucks.
It has been found that a dense, high purity aluminum nitride ceramic material having a room temperature volume resistivity lower than about 1xc3x971013 ohmxc2x7cm at a temperature of about 23xc2x0 C. can be produced by heat treating a hot pressed ceramic body consisting essentially of aluminum nitride in an atmosphere deficient in nitrogen.
The present invention is generally directed to a method of reducing the volume resistivity of a body consisting essentially of aluminum nitride. The method includes exposing the body to a soak temperature of at least about 1000xc2x0C. in an atmosphere deficient in nitrogen.
In another embodiment, the invention is a method for forming a polycrystalline aluminum nitride body having a volume resistivity less than about 1xc3x971013 ohmxc2x7cm. In this embodiment, a green body consisting essentially of aluminum nitride is sintered to form a polycrystalline body. The polycrystalline body is exposed to a soak temperature of at least about 1000xc2x0 C. in an atmosphere deficient in nitrogen for a period of time sufficient to cause the volume resistivity of the polycrystalline body to be less than about 1xc3x971013 ohmxc2x7cm at a temperature of about 23xc2x0 C.
In still another embodiment, the invention is a method for reducing the volume resistivity of an electrostatic chuck consisting essentially of aluminum nitride. In this embodiment, at least a portion of the electrostatic chuck is exposed to a soak temperature of at least about 1000xc2x0 C. in an atmosphere deficient in nitrogen.
The invention also is directed to an electrostatic chuck. The electrostatic chuck includes an electrode having a first side and a second side, and a body having a first portion at the first side of the electrode and a second portion at the second side of the electrode. The first portion of the chuck body has a volume resistivity less than about 1xc3x971013 ohmxc2x7cm. The volume resistivity of the second portion is within one order of magnitude that of the first portion.
The invention has numerous advantages. For example, the invention can produce polycrystalline aluminum nitride bodies having a density higher than 98 % of the theoretical density, a volume resistivity lower than 1013 ohmxc2x7cm at room temperature and which typically include no more than about 1000 ppm total metal impurities and no more than about 500 ppm carbon, and preferably, less than 490 ppm carbon. The bodies can be produced in the absence of sintering aids and have properties which render it particularly attractive in the fabrication of electrostatic chucks capable of operating in the Johnson-Rahbek mode. The electrostatic chucks of the invention can be manufactured in a reproducible manner and have a volume resistivity which is highly uniform throughout the chuck body.